Monthly Archives: November 2016

PDF URIs

I was handed an interesting PDF document. It doesn’t contain malicious code, yet it generates network traffic. Let me explain how this is achieved.

Creating a PDF that makes a HTTP(S) connection to a website is easy. There’s no need to use an exploit, not even JavaScript. You just have to use a URI object:

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On its own, this object will do nothing. An action is needed to have this URI requested. If you want this URI to be requested when the PDF document is opened, you could add an /OpenAction:

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Adobe Reader will not let this connection happen silently. The user will be prompted before the TCP connection (to subdomain.nviso.be in our example) is established:

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But even before the user clicks one of the buttons, Adobe Reader will do a DNS request for this domain (nviso.be):

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If the domain does not resolve to an IP address, Adobe Reader will do another DNS request for the subdomain (subdomain.nxdomains.be in this example, where nxdomains.be does not resolve to an IP address):

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In this case, the warning presented to the user is slightly different:

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This type of PDF document can be used to track users: when the document is opened, a DNS request is performed. If the request is a FQDN unique to the PDF document, then such a DNS request logged by the DNS server is a sure indicator that the PDF document has been opened. Remark that this DNS request will have a source IP address from a DNS server, not from the user’s machine.

If the user allows a connection to be made, then a TCP connection will be established between the user’s machine and the web server.

In a corporate environment with HTTP(S) proxies, the DNS requests can be prevented from going to the Internet.

Malicious Document Targets Belgian Users

In this blog post I want to show how a malicious document (maldoc) behaves and how it can be analyzed with free tools.

A couple of weeks ago many users in Belgium received an e-mail, supposedly from a courier company, informing them that a package was waiting for them (article in Dutch).

This is an example of the e-mail:

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This e-mail contains a link to a Word document:

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The Word document contains VBA macro code to download and execute malware (downloader behavior). But MS Word contains protection features that prevent the code from running when the document is opened in Word.

First of all, since the Word document was downloaded from the Internet, it will be marked as such, and MS Word will open the document in Protected View:

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The user is social-engineered into clicking the Enable Editing button. Because the Word document contains VBA macros, another protection kicks in:

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By default, MS Word disables macros for documents of untrusted sources. Only after the user clicks on the Enable Content button, will the VBA macros run.

The user is presented with an empty document, but meanwhile malware was downloaded and executed invisibly to the user:

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The VBA macro code can be extracted with a free open-source tool: oledump.py.

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When looking at the VBA code (streams 8 and 9), we find subroutine Document_Open in stream 9:

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This subroutine is automatically executed when Word opens the document. Subroutine Document_Open contains a call to subroutine TvoFLxE in Module1:

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Subroutine TvoFLxE removes the content of the document (this causes the document to become blank, see screenshot), saves the document and calls function HuEJcCj.

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In this function we see a call to CreateObject. This is always interesting and requires further analysis. CreateObject takes a string as argument: the name of the object to be created. In this code, the string is returned by function JFZzIeCKcjgPWI which takes 2 arguments: 2 strings that look like gibberish. We see this often in maldocs (malicious documents): strings are obfuscated, e.g. made unreadable. Function JFZzIeCKcjgPWI is a string decoding function, taking strings “MWqSBYcnRrviVpGRtY.ASJhGneqYlVl”and “FYqRnVNvJB1GqMA” and converting them to a meaningful string.

In this maldoc, the string obfuscation method is rather simple. Function JFZzIeCKcjgPWI removes all characters found in string “FYqRnVNvJB1GqMA” from string “MWqSBYcnRrviVpGRtY.ASJhGneqYlVl”. Was is left is string “WScript.Shell”. This Shell object can be used to make Windows execute commands. So we need to deobfus.

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When we deobfuscate these strings, we get this PowerShell command:

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This PowerShell command downloads an executable (malware) to disk and executes it. The downloaded malware seems to be ransomware, we’ll write another blog post if it has interesting features.

To protect yourself from this kind of attacks, never activate the document (Enable Editing and Enable Content). Anti-virus can also protect you by 1) detecting the maldoc and 2) detecting the executable written to disk. When you don’t trust a document, you can always upload it to VirusTotal.